In the initial year of the project, a miniature pH sensor, based on fiber optics, was designed to pass through a 22-gauge needle intended for implantation in tissue for physiological studies during exercise. The operation of the probe depends on pH changes identified by dye indicators. The probe is connected to laboratory optical instrumentation. During the second year, an electro-optic measuring and readout system was designed and developed specifically for the pH sensors. The new unit makes the overall system portable, practical, and easy to use. In vitro tests were performed to characterize the pH sensor and the instrument. In the current year, improvements in the probe were made, and its applicability to blood pH determination was shown.